1. Field of the Invention
The present invention concerns testing of electronic assemblies, and, more particularly, concerns testing of network communication by differential receivers and drivers to detect faults on electronic assemblies.
2. Related Art
IEEE standard 1149.6, Standard for Boundary Scan testing of Advanced Digital Networks, 2003, which is hereby incorporated herein by reference, specifies a way of finding faults at a high level of assembly on production lines, i.e., at a level above that of a chip or a module, such as at the printed circuit board assembly level. In particular, and referring to FIG. 1, a driver 103 and receiver 104 is shown on an assembly 100, which may be a printed circuit board or the like. Driver 103 and receiver 104 are referred to by the IEEE 1149.6 standard as a “mission” driver and “mission” receiver. Mission driver 103 and receiver 104 on assembly 100 are connected to a communication network 102 during production.
Previously, it has been conventional during assembly testing to use what is now called mission receiver 104 to detect faults on the assembly 100 portion of a communication network 102 to which receiver 104 is connected. However, because they are designed to communicate in a fault tolerant manner, the network 102 to which mission driver 103 and receiver 104 are connected may have a fault they do not detect, such as a high impedance due to a poor solder connection.
To address this problem, IEEE 1149.6 specifies certain assembly-related testing. (Because it's function is referred to merely as that of a “receiver” in the language of IEEE 1149.6, receiver 105 might be confused with mission receiver 104. Mission receiver 104 detects information on a pair of network conductors 102, and is independent of the other, fault-detection-related, so-called “receiver” 105. The fault-detection-related receiver 105 will herein be referred to as a “test receiver” 105.) Unlike a mission receiver, the IEEE 1149.6 test receiver is not intended to be fault tolerant. Quite the contrary, since it is solely for fault detection, test receiver 105 is intended to be fault intolerant. According to IEEE 1149.6, mission receiver 104 includes a differential receiver (not shown in FIG. 1) connected to a pair of network conductors 102. Test receiver 105 and mission receiver 104 are on the same chip 101. Mission driver 103 and at least part of network 102 are off of chip 101.
Referring now to FIG. 2, test receiver 105 includes a differential amplifier 110, but with only one of its differential inputs, IN—ACDC, coupled to only one of the conductors of network pair 102 (FIG. 1) and its other differential input coupled to a reference voltage IN—REF. In this manner, differential amplifier 110 is not assisted in proper detection of a transmitted signal on one conductor by a correspondingly transmitted signal on the other conductor of network conductor pair 102.
Also unlike a mission receiver 104 (and unlike most amplifiers, in general), the differential amplifier 110 of test receiver 105 has a preset circuit 112 and a hysteretic memory 114 that relate to operation in the following manner. Before a fault detection sequence begins, the preset circuit 112 of test receiver 105 drives a short pulse to initialize differential amplifier 110 to a predetermined state. Because of hysteretic memory 114, differential amplifier 110 tends to retain its initialized state even after its pre-set-circuit-driven initialization pulse ends, unless amplifier 110 is driven to a new state by signals on its inputs. After differential amplifier 110 is preset, mission driver 103 (FIG. 1) drives a sequence of test signals on network conductors 102. Differential amplifier 110 may or may not detect these signals, depending on whether there is an assembly fault. Because differential amplifier 110 is fault intolerant, a test signal on its input IN—ACDC from one of network conductors 102 succeeds in resetting the outputs of differential amplifier 110 only if the signal is cleanly transmitted, i.e., without being impaired by even a minor assembly fault, such as a slightly imperfect connection, for example.
While IEEE 1149.6 specifies an arrangement and manner of operation of the test receiver, it does so in general terms. Thus, a needs exists for a test receiver suitable for detecting faults on electronic devices during assembly testing and that meets the general specifications of IEEE 1149.6